# Tag Info

## Hot answers tagged acoustics

17

There are two things that can be considered: one is trivial - that it is quieter at night so you are more likely to hear the horn. The second is physics: the speed of sound depends on the square root of temperature, so the refractive index is proportional to $T^{-1/2}$. At night it is quite possible to get a temperature inversion, such that air near the ...

5

Yes. Higher frequencies are attenuated more over distance than lower frequencies are, which has a rounding effect on the square wave as the upper harmonics are reduced. Reference Do low frequency sounds really carry longer distances?

4

The trouble is that your table, or whatever object it is, will act as a waveguide. That's because the sound waves will (partially) reflect of the wood/air surface then travel back into the table and interfere with other waves. The result is going to be hideously complicated to calculate. As Luboš says in a comment, if the thickness of the table is much less ...

4

TL,DR: Magnetic coupling results in lower transmission of sound energy than physical contact Controlling what surfaces vibrate gives more control over sound generation The same benefit could be achieved with other forms of isolation (e.g. foam) but it wouldn't look as cool. It is bunk, mostly. A magnetically levitating speaker maintains a ...

4

The different tonality of a note in different instruments stems from the different mixes of amplitudes in the harmonic frequencies that the instrument provides. To be more concrete (and keeping to a slightly simplified view), you play the A note (440 Hz) and then you have the harmonic frequencies 880, 1320, 1760, ... ($440n$ where $n$ is the number of the ...

4

The explanation of alemi is correct. I noticed this in 2006 and had a first-year do a study of it: http://www.math.dartmouth.edu/~ahb/notes/Paopao2007poster.pdf (the spectrograms are not good in that poster). Here's a spectrogram we recorded from a hand-clap in the standard 20'x20'x40' court showing two clear slopes: We also found foot-stamping as a good ...

4

It might not have to do with day/night physics at all. It is possible that the train doesn't run during the day so it is unlikely you would even hear the horn. I live in a city, about 2.5 miles from the rails, where during the day the train tracks are used only for public light rail transit. The large locomotives only run at night after the light rail ...

4

In principle, sure. That's what microphones are, as ACuriousMind points out. But if you want to power anything substantial, an important issue to overcome is the relatively small amount of energy contained in sound waves. According to this website, the front rows of a rock concert have a sound intensity of $10^{-1}~\text{W m}^{-2}$. So even if you had a ...

3

There are several explanations, depending on a lot of factors. At night, the air near the ground can have a different temperature than air only a few hundred feet above1. This affects the transmission of sound waves. There is often less wind at night. If the wind is blowing towards the train it can be harder to hear There is usually less ambient noise ...

3

It's not just a pure single frequency of sound that is being transmitted by an instrument. Just like with light, if you ask the frequency of the sun's emission, the answer would be that it's a whole broad spectrum (hence its ability to produce a rainbow, or allow objects to reflect colours other than yellow) but its peak frequency is yellow. You can ask for ...

3

For a long time, timbre was believed to be based on the relative amplitudes of the harmonics. This is a hypothesis originally put forward by Helmholtz in the 19th century based on experiments using extremely primitive lab equipment. E.g., he used Helmholtz resonators to "hear out" the harmonics of various sounds. In reality, the relative amplitudes of the ...

3

If the bell is still vibrating when you let air inside it, then the answer is yes. If the bell was damped just before the door is opened, then the answer is no. Sound is transmitted through compression / decompression waves (pressure waves) in a medium (e.g. air, water, wall). This necessitates contact of the vibrating source of sound with such a medium. ...

3

As long as your music player is traveling with you, the music would slow down (as seen by a stationary observer). But you would not notice anything different. That's the beauty of relativity. Your "internal" clock would slow down as much as the music player's clock, and they remain relatively in synch.

3

This advertising strategy is basically using pseudoscience to get naive people to buy a product. The efficiency problem in speaker design has nothing to do with momentum transfer from the speaker to the air. That's trivial, since the mass of air a speaker moves is typically orders of magnitude less than the mass of the speaker itself. Instead, the (low ...

2

Speech sounds can be either periodic, like "aaah," or nonperiodic, like "sh." Periodic means that the pattern repeats over and over with a certain frequency. Here's a graph of sound pressure versus time for me singing the vowel "ah" at a fixed pitch: This kind of graph is referred to as a "time domain" representation of the sound, because it has time on ...

2

In today's news - researchers at MIT did just that using high-speed camera with frame rate between 2 kHz and 6 kHz. They used some advanced filtering to detect microscopic movement of objects, but for details we will have to wait until they publish their paper.

1

It seems that the harmonic (integer multiple) overtones of a sound usually all have the same phase. Is this true...? No, I don't think this is generally true, although it may be true for certain instruments. What led you to believe this? In trumpet tones, for example, the different harmonics come up at different times during the attack, so it seems ...

1

Paper cones were originally chosen for their rigidity and lightness, so they can move air quickly without deforming and couple to a motor easily at the center while also being easy to suspend from the basket by their perimeter with a simple corrugation or foam/rubber surround. Physics only played a major part in the ease of construction and performance was ...

1

The energy flux of an acoustic wave is $$\vec J = \vec v p \;\;\;\;\;\;\;\;\;\;\;\;\; (1)$$ The relevant energy density to be used in these calculation is actually $p+1/2 \rho v^2$, but since we are discussing a small amplitude wave (= no shock wave), $v$ is an infinitesimal quantity; thus $1/2\rho v^2$ is lower order than $p$ (second vs. first), thus it ...

1

The continuous stream of air that you are blowing in, it doesn't enter the pipe continuously. When the stream of air hits the hard edge in an organ pipe, it flaps in and out of it due to the difference in the density of the air outside and inside the pipe. This oscillation of the air in and out, it will be a periodic energy supply for the standing wave in ...

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